The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In order to enhance fuel efficiency of automobiles, car bodies and chassis are desired to be reduced in weight, and the most effective method for reducing a weight includes a scheme of using a material having a low specific gravity or a scheme of reducing a thickness of a material compared with an existing material.
However, the scheme of using a material having a low specific gravity has shortcomings in that cost is significantly increased, and the scheme of reducing a thickness of a material compared with an existing material is easy to secure strength through application of an ultra-high strength steel material but involves weak rigidity.
In the scheme of reducing a thickness of a material, rigidity may be reinforced by changing a design structure of a product. In this case, however, a shape of a product may be complicated and, in particular, application of an ultra-high strength steel material has limitations in processing a product having a complicated shape due to low formability of the ultra-high strength steel.
Any one of a general die and a progressive die is selectively used in a forming method using the ultra-high strength steel material.
The forming method using the general die is disadvantageous in that it has low productivity as each unit process using several dies is individually performed.
In contrast, we have discovered that in the forming method using the progressive die, a plurality of unit processes (forming, bending, shearing, and piercing, etc.) may be continuously performed through sequential transfer in a single die, obtaining high productivity and being appropriate for mass-production to reduce cost.
However, the progressive die has limitations in forming a product having a complicated shape, and in particular, when a material having low formability such as an ultra-high strength steel material is applied to the progressive method, cracks may easily occur due to low formability of the material and machining accuracy and forming quality are significantly degraded.